Ghost shrimps are best. They cheap 20cents a pop and will also eat fish feces (yes I didn't believe this at first until I noticed my shrimp chowing on poo one time). Only risk, dependent on betta personality is they may think of them as food. In this case improve the odds of the shrimp survival rate by adding deco or plants as hiding spots. On the rare chance your shrimp becomes a meal to your betta you will be happy to know it was only 20 some odd cents down the drain and your betta got himself a nutritious meal.